12 December 2001
Dredging accuracy tolerances
Discussions of this subject are contained in various sources including the joint PIANC
and IAPH Final Report entitled, "Approach Channels - A Guide for Design," PTC II-30,
dated June 1997; the Report of Ship Channel Design (ASCE, 1993); Chapters 3.2.1
and 8.3.1 in Dredging: A Handbook for Engineers (Bray et al., 1997).
Overdepth Dredging. Previous studies have shown up to 20 percent of
over excavation on typical dredging projects. In addition to the additional dredging
costs encountered, this can also result in premature filling of scarce disposal areas.
Two factors that control overdepth dredging are dredge control and dredge survey.
Modern dredge control is achieved by instrumentation, which provides real time
knowledge of cutterhead depth and velocity and density of the slurry being pumped
(Herbich, 1992). However, in some situations, it is less expensive to overdredge an
area by 0.3 or 0.6 m (1 or 2 feet) than to pay for the careful manipulation of dredging
equipment and for the extra time involved in dredging to the exact depth required.
Overdepth dredging also allows for some additional shoaling before dredging is
required again. Overdepth dredging should be investigated for each specific site as it
cannot be used in every situation. For example, it may jeopardize stability of nearby
Advanced Maintenance Dredging. It is advantageous to minimize
maintenance costs by performing advanced maintenance dredging to increase the
interval between dredgings. The channel is dredged deeper and/or wider than its
original design or authorized depth, thereby allowing for shoaling to occur while the
channel can still be navigable. Advance maintenance dredging can also greatly
increase the time intervals between maintenance dredging operations, which can
reduce the number of mobilization and demobilization tasks and the costs associated
with them. This method is economically feasible if the money saved through a reduced
dredging frequency schedule is greater than the cost of removing any additional
sediment that may have been deposited as a result of the deeper and wider channel.
To determine whether or not advanced maintenance dredging is
economically feasible, the design engineer must analyze and extrapolate shoaling data
for "prior to dredging" depths of the channel and use numerical models to simulate site-
specific estuarine hydrodynamics and sediment transport mechanisms. If appropriate
data are unavailable for performing analytical studies, and numerical modeling studies
are cost prohibitive, a minimal advance maintenance effort can be put into effect and
the results evaluated. Thus, if money is saved, advanced maintenance dredging efforts
can be stepped up and, conversely, if money is lost, the efforts can be abandoned
(Sedimentation Control To Reduce Maintenance Dredging of Navigational Facilities in
Disposal Areas. The selection of an appropriate disposal area depends
upon the physical characteristics of the dredged material, potential environmental
impacts, size of the project and social, political, economic and regulatory
considerations. Dredged material disposal sites may be offshore or onshore sites,